A typical supplemental inflatable restraint activation circuit is shown in FIG. 1. The circuit is provided with power source 10 comprising vehicle battery 12 supplying power through diode 14 and comprising storage capacitor 16 providing power through diode 18. Storage capacitor 16 is used in the event vehicle battery 12 is disconnected during a deployment situation. During a sudden deceleration of the vehicle in which it is desirable to deploy the supplemental inflatable restraint system, electromechanical sensors 20 and 24 close, closing the current path between power source 10 and ground through squib 22, which is an ignitable initiator that causes inflation of the supplemental inflatable restraint.
In the systems of the type shown in FIG. 1, when the sensors 20, 24 close during a sudden deceleration such as would require deployment of the supplemental inflatable restraint, the current that passes through the sensors is limited only by the electrical resistance of squib 22 and the energy input from power source 10. The result of the high current flow through sensors 20 and 24 causes arching of the contacts as the sensors' switches close. This arcing destroys the switches by damaging contact surfaces so that the switches cannot be re-used. If electrical type switching devices are used, the high current initiated through the electrical switching devices can destroy those devices.
What is desired is a suitable controlled current source for use during deployment of a supplemental inflatable restraint.